Digital cameras are typically equipped with auto-focusing systems. These systems typically have motor driven lenses that are adjusted to a position that corresponds to the maximum of a focus signal. A focus signal is a signal that has a signal strength proportional to the closeness to focus of the focusing system (the better the focus the higher the signal). The focus signal is typically calculated using the contrast method. See U.S. Pat. No. 5,597,999, which is incorporated by reference, for an example of generating a focus signal using a contrast method. Other methods of generating the focus signal are available but are not generally used in digital cameras. Focus signals are typically symmetrical along the optical axis of the focusing system (i.e. an object too near the optical system and an object to far from the optical system will both give a lower signal strength than an object at the proper focus).
For example, figure one shows a typical focus signal 102 with the maximum signal strength at point 104. The X-axis 110 is the physical distance of the object, to be focused, along the optical axis of the system. The Y-axis 112 is the signal strength of the calculated focus signal. Locations 106 and 108 are displaced from the maximum focus signal by distances 114 and 116. Location 106 is too close to the lens system to be in optimum focus and location 108 is too far from the lens system to be in optimum focus. Points 118 and 120 give the focus signal strengths at locations 106 and 108 respectively and the signal strengths are substantially equal. Because the signal strengths of location 106 and 108 are substantially equal the automatic focusing system can not differentiate between the two locations. The system can not determine, just by the signal strength of the focus signal, if the object to be focused is too close or too far from the optical system. Because it can not be determined if the object to be focused is too near or too far, the system will not know which way the lens should be moved to bring the object into focus.
There are two typical solutions to this problem. The first solution is to always start the lens at one of the positions where the lens is at its maximum movement (i.e. at the farthest or nearest lens position) and then move the lens towards the other maximum lens location until the system is in focus. This method has a number of problems. The first problem is that the total system movement for the lens can be large. For example, when the initial lens position is set to focus things at large distances and the object to be focused is very near the camera, the lens will travel almost its full movement range before the object is brought into focus. The second problem is that each time the system is to be focused the lens must be reset to the initial starting location.
The second typical solution is to set the lens in the middle of the travel range and calculate an initial focus signal strength and then move the lens some amount in one direction and calculate a second focus signal strength. Using these two calculated signal strengths the correct direction to move the lens for proper focus can be determined. This method will sometimes move the lens in the wrong direction to calculate the second focus signal strength. When the system does move the lens in the wrong direction to calculate the second focus signal strength, the lens must be moved in the direction back towards the initial location to bring the system into focus. This takes additional time.
Auto-focusing systems are typically one of two types. The first type allows the lens to be adjusted to any point between the maximum and minimum lens position. The second type has a limited number of set positions where the lens can be located.
Color digital cameras typically use area sensor arrays. These area sensor arrays are typically composed of a plurality of sensor elements or pixels in a two dimensional array. These pixels are typically covered by a set of red, green and blue filters arranged in some pattern, typically a Bayer dither pattern (see FIG. 2). The color digital camera records the information for each color using the area sensor elements covered by that color filter (i.e. the pixels covered by the red filters produce the red color signal). Because the Bayer dither pattern has twice as many pixels covered by a green filter compared to either the red or blue filters, the effective sampling rate for the green color is twice the sampling rate for either the red or blue colors. Because of this higher resolution, color digital cameras typically use the green color to calculate the focus signal. Other colors or a combination of some or all of the colors could be used to calculate the focus signal. Because the area sensor array has all the pixels or elements substantially in the same plane, the lens of the camera is typically designed to focus all of the colors in that same plane.
Lens systems typically consist of a number of optical elements. Each optical element can be made from a range of optical material. A given optical material is typically identified by the speed of light in the optical material compared to the speed of light in a vacuum (i.e. its refractive index N). The refractive index of an optical material is used to calculate the angle change a ray of light makes when entering or leaving that optical material. The refractive index of optical materials is dependent on the wavelength of light. Because the refractive index is dependent on the wavelength of light, a single element lens that focuses green light at one location will focus blue light at a different location. This difference in the focal point of different colors is called primary chromatic aberration. The dependence of the index of refraction on wavelength for a given optical material is defined by the dispersive power or Abbe number V and is calculated as V=(Nb-Nr)/(Ny-1) where Nb, Nr, and Ny are the index of refraction for the given optical material at blue, yellow, and red light respectively. Different optical materials have different Abbe numbers. A lens system with two or more elements, where each element is made with a different optical material, can be designed to eliminate primary chromatic aberration. Achromates are lens systems, with two or more elements, designed to eliminate the primary chromatic aberration, typically by making the focal lengths for red light (656 nm) and blue light (486 nm) the same. There will be some chromatic aberration at wavelengths between the two corrected wavelengths, but it is typically small and called secondary or residual chromatic aberration. Lens systems can be designed to eliminate primary chromatic aberration at more than two wavelengths.
There is a need for a reduction in the amount of time required for auto focus for color digital cameras and for less expensive lenses.